1. Field of Invention
The present invention relates in general to a distributed temperature sensing method based on the spontaneous Brillouin scattering effect, more particular, to a spectrum decomposing method to achieve high spatial resolution, high temperature resolution and long sensing range of the distributed temperature measurement.
2. Description of Related Art
There are two ways to fulfill the distributed sensing approach. One includes the use of single sensors being discretely arranged along a sensing line, but will make the whole sensing system much complicated. The other one as described hereinafter includes the use of optical-fiber sensors to obtain the detecting physical parameters along a linear fiber depending upon the optical characteristics thereof. Under the circumstances, the optical fiber are regarded, on one hand, as an active component for sensing measurement and, on the other hand, as a passive component for the information transmitting material to obtain the following advantages:
1. The optical fiber is small in volume and light. Thus, the optical fiber can be adopted easily anywhere.
2. Since the frequency bandwidth of the optical fiber is large, many signals may be transmitted simultaneously.
3. Since the optical fiber is made of nonconductive insulating material, it is not influenced by external electromagnetic waves. Thus, the signal may be transmitted without noise.
4. Due to the development of optical fiber technology, optical fibers can be manufactured at a low cost.
As such, the utilization of fiber-distributed sensing for the measurement of strain and/or temperature distribution is widely applied on many implementations to monitor such as tunnels, bridges, dams and airplanes, buildings and etc. for safety-secured purpose.
Recently, the distributed temperature sensors (DTS""s) that use Brillouin scattering as the sensing mechanism have been intensive studied. The Brillouin frequency shift is dependent on the temperature and strain conditions of the optical fiber, which provides the basis for a sensing technique capable of detecting these two parameters. In the Brillouin-based distributed temperature sensing system, if a higher spatial resolution is accomplished, the measured temperature distribution is more closed to the practical situation of the fiber. The sensing spatial resolution is defined as the 10%/90% rise times from the unheated section to the heated section of the fiber. To achieve higher spatial resolution in a Brillouin scattering system, the measurements utilizing a short-pulsewidth laser source have been reported, which are disclosed by T. Horguchi, K. Shimizu, T. Kurashima, M. Taleda, and Y Koyamada, published in J. Lightwave Technol., 13, 1296 (1995), and A. Fellay, L. Thevenaz, M. Facchlni, M. Nikles, and P. Robert, published in Proc. OSA Tech. Dig., 16, 324 (1997). However, owing to the Brillouin linewidth limitation, it is obvious that using the time-domain pulsed approach is unsuitable for distributed measurements of submeter spatial resolution unless other techniques are employed. More recently, several methods have been reported for performing the measurement with submeter spatial resolution. One such technique, disclosed by K. Hotate and T. Hasegawa, published in Tech. Dig. Opt. Fiber Sens., 17, 337(1999), is the direct-frequency modulation method that demonstrated a sensing spatial resolution of 45 cm over 7.8 m sensing range, and another techniques, disclosed by M. D. DeMerchant, A. W. Brown, X. Bao, and T. W. Bremner, published in J. Lightwave Technol. 38, 2755 (1999), and A. W. Brown, M. D. DeMerchant, X. Bao, and T. W. Bremner, published in J. Lightwave Technol. 17, 1179 (1999), utilize the sensing fiber with uniform strain and identical length in each section to achieve the spatial resolution of 40 cm and even 25 cm with the enhancement of compound spectra processing method. In addition, a Brillouin-based distributed temperature sensing system that provide a spatial resolution of 35 cm and a temperature resolution of 4.3xc2x0 C. over 1 km based on measuring the Landau-Placzek ratio with a pulsewidth of 3.5-ns has also been reported by H. H. Kee, G. P. Lees, and T. P. Newson, IEEE Photon, Technol. Lett., 12, 873 (2000). However, the short-pulsewidth laser sources are requisite for these methods to accomplish measurements of submeter spatial resolution. Thus the sensing ranges of these methods are limited.
It is therefore, in one aspect, an object of the present invention to provide a method that can provide a distributed temperature measurement with high spatial resolution and long sensing range based on decomposing the spectra of the spontaneous Brillouin scattered signals. This method utilizes a long-pulsewidth laser source to derive the long sensing range and employs a signal processing technique of decomposing Brillouin spectrum to raise the spatial and resolutions to submeter level.
According to the above-mentioned objects of the present invention, the method for distributed temperature measurement based on decomposing spectra of spontaneous Brillouin scattered signals includes: (a) supplying a laser source with an optical pulse to an optical fiber; (b) obtaining a first measured Brillouin spectrum in a reference temperature section of the optical fiber, and at least a second measured Brillouin spectrum and a third measured Brillouin spectrum in a temperature overlapped region of the optical fiber, the measured Brillouin spectra above corresponding to the optical pulse entering a fiber section of the optical fiber with a length of d at a traveling time td for ti greater than td greater than t0 and a sampling interval t1xe2x88x92t0; (c) determining the length of d according to the measured Brillouin spectra above and a weighting factor ranging from 0 to 1; (d) determining a real Brillouin spectrum profile of the fiber section according to the length of d, the corresponding weighting factor and the measured Brillouin spectra above; and (e) determining a temperature distribution according to Brillouin frequency shifts of the real Brillouin spectrum profile.
As a result, a spontaneous Brillouin-based distributed temperature sensing system using a new Brillouin spectrum decomposing technique to achieve high spatial and position resolutions, high temperature resolution and long sensing range. For a 9500-m sensing range of standard single-mode fiber and a 100-ns pulsewidth laser source, a spatial resolution of 20 cm and a temperature resolution of 1xc2x0 C. are simultaneously achieved by using this signal processing method.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.